Large pipelines that supply water or transport sewerage often traverse undulating terrain. This is one reason why the proper operation of these pipelines requires the removal of air pockets that may form during operation or during the pipeline filling process. Similarly, pipelines occasionally experience negative pressures that may be due to emptying, pumping disruptions, and maintenance or failure conditions. Regardless of the reason, large diameter pipelines are prone to damage under negative pressure conditions and venting valves that allow the ingress of air can be a necessary design requirement.
The prior art shows and describes several types of pipeline venting valves. Certain of the prior art valves utilize small floats that operate the valve closure using mechanical advantage offered by sets of hinges and levers as exemplified by U.S. Pat. Nos. 4,114,641; 4,635,672; 5,090,439; 5,386,844; 5,988,201; 7,617,838; and 4,209,032. Other prior art valves avoid the maintenance problems associated with levers by using larger direct acting floats as exemplified by U.S. Pat. Nos. 5,511,577; 2,853,092; 4,579,140; 4,586,528; 4,696,321; 4,742,843; and 5,769,429, or have multiple venting modes that allow large flow rates of air into and out of the pipeline as exemplified by U.S. Pat. Nos. 5,511,577 and 6,513,541. Still certain other of the prior art valves do not allow the ingress of large volumes of air into the pipeline under negative pressure conditions while other valves vent only small amounts of air that accumulate under normal pipeline operations.
During operation of prior art cylindrical type valves such as that described in U.S. Pat. No. 5,511,577, or as depicted in FIG. 1, the normal operating liquid level in the valve is just high enough to supply the control float with sufficient buoyancy so as to apply sufficient pressure to seal some nozzle or gas release mechanism. This creates a “normal gas pocket volume”. If the level drops, due to accumulation of additional gases from the pipeline to which the valve is attached, the control float will drop slightly and unseal the nozzle, thus releasing some gas until the liquid level increases to again establish a sealed condition. A similar scenario occurs if the pipeline pressure happens to momentarily drop, which would induce an apparent expansion of the gas pocket that would similarly unseat the nozzle seal. However, when the pipeline pressures surge from a minimum operating design pressure up to the maximum design pressure, this gas pocket will compress (called the “compressed gas pocket volume”) in approximate ratio to the two pressures. When this happens, it is desirable to keep the rising liquid surface below the valve's sealing surfaces, so as to prevent debris from inhibiting an effective seal. To ensure a sufficiently tall “compressed gas pocket volume” is still present at this maximum pressure, the ratio of the “normal gas pocket volume” to the “compressed gas pocket volume” needs to approximate the ratio between the minimum and maximum gas pressures. In a valve with a constant cross-section, this geometric requirement results in the valve becoming quite tall, especially with working pressures of 10 or 25 atmospheres.
A problem with tall valves is that these valves are generally installed underground in pipework vaults and manhole covered access chambers that have limited headroom. Due to the limited headroom, engineers can be forced to install vent valves that are shorter and under capacity for the duty required, or may elect to install a vent valve at another, less optimal location.
One prior art solution is to replace the straight cylindrical body of the valve with one having a greater internal diameter and then weld on a pipe reducer to the top and bottom as depicted in the prior art valve in FIG. 2. This effectively generates a smaller volume at the top of the valve (where the nozzles and seals are located) and a larger air volume beneath it, and this combination produces a higher maximum pressure valve or enables the valve to be designed shorter. However, this is costly and normally reserved for valves with pressure ratings of 15 bar or above.
Another solution is to utilize a valve that uses internal levers to create some mechanical advantage, rather than depending on direct float buoyancy to affect a seal. While the use of levers can result in a shorter valve, these levers are subject to fouling by debris and breakage.
The prior art thus perceives a need for a gas vent valve that allows gases in large diameter water and sewerage pipelines to be released when the pipeline is pressurized or filled and allows air to enter into the pipeline when the internal pressure of the pipeline drops below atmospheric pressure. Further, the prior art perceives a need for a gas vent valve that can be installed in situations with less headroom and still maintain a high venting capacity and the reliability advantages of direct acting floats and seals.